All fans spin but NO POST

After intensive search in this forum I’m posting my problem…
I'd assembled a System in Oct 04 with OS Win2K. It worked well for an year. I decided to upgrade the OS to WinXP. While upgrading I noticed during one of the reboots that the welcome page before POST began, had shown as Celeron D, whereas the system I had built was Pentium IV 2.4Ghz. .. Anyway, I completed upgrading the System to WinXP. During subsequent restarts, I observed that at least 1 out of 5 times welcome page showed as Celeron D. Out of curiosity, I opened the heat sink to reconfirm the make of the CPU. It was marked as Pentium IV 2.4GHz. Reassembled everything back.

Now, I started facing the series of actual problems: :unch:
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1. The System works well, as long as I run ordinary programs.
2. Idle conditions, the CPU zone temp is 46 to 48C max. Any low end games will raise the CPU temp to 54 to 56C max. And high end games will raise temp above that & system will freeze!
3. Well the system can be used for several hours in idle condition or with ordinary software, but the system DOES NOT START even after normal shut down..
4. After normal shut down and wall power taken off, when the system is powered back again, the CPU & Cabinet fans run and NO DISPLAY, NO POST, NO BEEP....
5. I tried all possible methods to identify the problems through reading the Intel website, Forums etc...
6. Some of the quick solutions were like, CMOS battery change, Bios recovery, RAM replacement, PSU replacement and reassembling the parts one-by-one in sequence, etc. Nothing worked.
7. Finally, I could start the system by removing the CPU & reseating it again, placing the heat sink back with thermal paste. But the problem starts again when the system is restarted. But the problem is solved temporarily every time by reseating the CPU! And no other methods mentioned in sl. no. 6 work

My first question is, which out of CPU & MB has gone bad or both of them? And next Q is, Can the problem be resolved in any tricky method or should I try returning the CPU/MB Intel?

Did yuo install the motherboard drivers after installing XP? also what brand of PSU is it you have? 300 watts is pretty weak for a P4 system. the very first one I built was a 2.6Ghz P4 and the case I had purchased had a codegen 300 watt psu and the system wouldnt even boot with it.

To ISS:-
The problem is existing before upgrading to Win XP.
PSU is good one (VIP), anyway i've swapped to 400 watts, still no joy.
And do u think the PSU should give ANY INTERMITENT problem?

To Eric Legge:-
Got BIOS update from Intel website.
I couldn't get ur second line...I've Pentium 4 which is seem marked on the CPU as well as i had alreay got it identified throu several other softwares like CPU-Z.
Well i like that suggetion of checking the processor and the socket with a magnifying glass. I've added in my next checklist...I shall come back with that report..

yes a dodgey PSU you most certainly cause intermintent problems and all kinds of bizzare computer problems. many people dont realize that a PSU with unstable voltages can cause all kinds of Hardware AND software problems.

I agree with u for PSU giving INTERMITENT problem. And that is the reason I had replaced with brand new PSU (VIP 300 Watts) but no joy in my case..

But, the CPU cannot give any INTERMITTENT problem. I'm really sorry I intended to ask about CPU and not PSU in my last posting, replying 2 your post. That was my typing mistake, I should always preview the the post before uploading...

Yes, I suspected it could be MB as u said.
Swapping the components with other best known components is the best way to detect & eliminate the problem. I dont hv a spare MB right now. I shall wait 4 sometime to get one or trade 4 it. Anyway, I shall come back with the result when do the test.